A bus in a CMOS system usually comprises a plurality of bus lines. A plurality of devices may be connected to each one of the bus lines. The devices connected to the bus line may have outputs which can be in any one of three possible states. The first of these states occurs when the output of the device is a low impedance path to the lower voltage rail (usually circuit ground), a condition commonly referred to as a logic "0" state. The output of the device is said to be in a second state if the device exhibits such a high impedance at its output that, as far as the other devices connected to the same bus line are concerned, it does not exist. The third possible state is when the output of the device is connected via a low impedance path to the upper voltage rail, a condition commonly referred to as a logic "1" state.
A CMOS bus line usually has connected thereto a number of buffer drivers for driving the bus line to one of the possible two low impedance states. If a bus line is not being driven by any one of the drivers for a protracted period of time, the bus line will start to drift from one particular logic state toward the other because of the inherent capacitance and leakages to ground and/or the upper rail. If the bus line is not held to-either the drain or the source supplies, i.e. V.sub.dd or V.sub.ss, the bus line could eventually drift into the range between 30 percent to 70 percent of (V.sub.dd -V.sub.ss), the so-called "forbidden zone." It is in this voltage zone that both the P and N channel input transistors of a CMOS gate connected to the bus may be caused to conduct simultaneously, as the bus line is floating between the two rails. This results in excessive current flow through and subsequent overheating of the input transistors connected to the bus line. A conventional method of correcting the aforesaid problems is to tie either a pull-up or a pull-down resistor to the bus. However, a tying resistor may cause a bus line to transit too slowly through the "forbidden zone," may drain too much power and may prevent the bus from being driven completely to a rail voltage.